The present invention concerns die punches, and more particularly concerns a die punch having means for indicating if the tip of the die punch has broken.
Punches are commonly used in dies to form holes in sheet metal during stamping operations. Periodically, the tip of a punch will break and all subsequent stamped pans will be defective until the broken punch is discovered and the stamping operation is stopped. If the problem is quickly discovered, only a few defective parts will be made. However, with automated die operations such as progressive sheet metal dies, pick-and-place dies, transfer dies, and similar die arrangements, the problem of a broken punch may not be discovered until several racks or bins of defective parts are made. Even if the defective pans can be reworked, reworks are labor intensive and inefficient, and part quality is difficult to control. As a result, the defective parts are often thrown away or otherwise disposed of. Quality control sensors can be used to assure that holes have been properly formed in stamped parts. However, sensors that are separate from the punch or die often give false readings since they depend upon the accuracy of part placement after stamping, and also since they depend upon the durability of the sensor which can be physically abused over time by moving stamped parts. Further, sensors require a separate station and require separate maintenance, both of which are undesirable and add expense. Still further, sensors located after the stamping operation are not an integral part of the stamping operation, and thus are "add on" features that take up space and can be maintenance problems.
U.S. Pat. No. 3,728,919 (to Scott) discloses a die punch including a pressurized passageway, and a pressure differential switch connected to the passageway to sense pressure change if the punch breaks. The pressure differential switch is operably connected to a press running a die incorporating the die punch to sound an alarm or otherwise stop the press if the die punch breaks. However, this arrangement requires that a compressed air hose and electrical wires be operably connected to the switch and to the press. Often there is often not a lot of room to position air hoses in dies. Further, air hoses are subject to being damaged, worn, or pinched such that they require maintenance. Still further, differential pressure switches are sensitive to contamination such as dirt, oil and other debris, which contamination is likely to be found around a die, either during operation of the die and/or during storage of the die. Additionally, modern press controls are usually electrically based, and thus a pneumatically based system is not preferred.
German Foreign Patent 329,753 (to Kischkat) discloses a die arrangement including a die having a punch holder, a punch attached to the punch holder, and a compression plate spaced from the punch holder for holding stock sheet material during the punching operation. A contact (h) on the punch engages a sensor (g) on the compression plate and establishes an electrical circuit with the punch during a "down" cycle of the press (FIG. 2). If the punch tip is broken, the punch does not make contact with the sensor, thus indicating a problem. However, the punch tip must break far enough up the shaft of the punch to prevent the punch contact (h) from engaging the sensor (g) before the sensor is no longer engaged on each press cycle. Further, sensor (g) is a separate part not incorporated into a punch per se, and thus it requires separate maintenance and adjustment. Still further, the disclosure in German '753 requires a particular die arrangement including a compression plate having room for the sensor. This arrangement is not compatible with many existing die structures and arrangements, and thus the disclosed arrangement is not retrofittable to many existing dies.
U.S. Pat. No. 3,124,026 (to Loeffel) discloses a spring biased misfeed detector including a pilot for progressive dies. A safety circuit in the tail section of the pilot is operably connected to a material feeder motor so that when the pilot engages misfeed stock material, the pilot is unseated and stops current flow to the feeder motor. Notably, the pilot disclosed in patent '026 can be used to sense the absence of a hole, but is not constructed to punch a hole, nor is it constructed to indicate if a punch tip has been broken.
Canadian Foreign Patent 666,057 (to Haywood) discloses a shear pin including a longitudinally extending passageway and a probe extending partially into the passageway and axially centered therein. If the shear pin breaks, the probe contacts the sidewalls of the shear pin to indicate the shear pin has broken. However, the structure of the shear pin and also the supporting structure are unsuitable for a die punch. Notably, the shear pin is intended to break when high loads are encountered. For example, the shear pin includes shear notches (75). Also, the proximity of the probe to the sidewall of the shear pin may lead to false readings.
Thus, a tool breakage sensor solving the aforementioned problems is desired.